Power distribution circuits typically used by electric utilities can experience various faults that disrupt service to consumers of electricity. Causes of such faults include electrical insulation breakdown or mechanical failures. Most commonly, faults are manifested as short circuits from line to ground, but line to line short circuits and open circuit faults can also occur. When these events occur, safety devices, such as circuit breakers, can be automatically actuated to shut down the distribution circuit. It is important for the utility and its customers to have the problem located and repaired as quickly as possible so that electrical service can be restored with minimal down time. In practice, because power transmission or distribution circuits extend over large distances, repair crews must patrol the entire line section. Locating faults is thus presently time consuming and expensive due to the lost revenue and the cost of lengthy troubleshooting.
Systems known in the art for localizing power line faults include stand-alone (independently-functioning) fault detection devices deployed on distribution feeders that emit light or audible alarm signal when they detect an abnormally high current magnitude. This approach, however, does not eliminate the need for repair crews to examine long stretches of power line as part of troubleshooting a line fault. There are also other devices known in the art that can be equipped with contact outputs for use with a supervisory control and supervision (SCADA) system, but no integrated system approach exists to date for localizing faults potentially occurring over a long stretch of power line.
Other systems known in the art rely on a variety of techniques for identifying and localizing power line faults. These techniques include methods of wave modeling, impedance evaluation, signal injection, and multi-phase analysis techniques. For example, U.S. Pat. No. 6,879,917 discloses a double-ended distance-to-fault location system using time-synchronized positive- or negative-sequence voltage and current measurements from both ends of an overhead transmission line to determine the exact distance to a fault with respect to either end. U.S. Pat. No. 6,924,647 discloses a fault location method and device, wherein the method includes the step of measuring the apparent impedances of impedance relays at line terminals at each end. U.S. Pat. No. 6,525,543 discloses a fault type selection system for identifying faults in an electric power system. The fault identification system includes a first logic circuit which is responsive to conventional protective elements which recognize the presence of low resistance single line-to-ground faults for the A, B, and C phases on a power transmission line. Other systems and methods are disclosed in U.S. Pat. Nos. 5,428,549; 6,415,244; 6,477,475; 6,756,786; 6,822,457; and 6,917,888. All of the aforementioned patents are incorporated herein by reference.
Another problem experienced by utilities involves a reverse electromotive force (EMF) generated by certain loads in a power outage. Motorized equipment and appliances (air conditioners, for example) that are loads on a broken distribution circuit can, following a fault, generate an EMF in the broken circuit section that appears as a flow of power from the load in a direction opposite the ordinary flow of power during normal system operation. For example, in the case of a line fault affecting one of three phases of a power distribution circuit, a three-phase motor powered by the remaining two phases will continue running, and may well operate as a power generator producing a reverse EMF back onto the wire of the faulty phase. Measuring line current magnitude, alone, near the fault point, is therefore sometimes insufficient to distinguish between an actual fault and a momentary imbalance of the distribution circuit.
Other drawbacks associated with conventional current measuring devices include the hazards associated with installing these kinds of devices on high voltage power lines. Therefore, a need remains in the industry for an efficient and effective fault monitoring system.